The Fabry-Perot Interferometer (FPI) is described by C. Fabry and A. Perot in 1897 (Ann. Chem. Phys., 12:459-501) and is widely used in a variety of applications of optical systems. The basic structure and operation of the FPI sensor is well-known in the art and is described in many physics and optics texts. This interferometer includes an optical cavity formed between two typically reflecting, low-loss, partially transmitting mirrors.
The use of optical fibers allows for the manufacture of extremely compact and economic sensors known as Fiber-optic Fabry-Perot interferometric (FFPI) sensors. Ends of optical fiber portions form partially reflective surfaces with the cavity or gap therebetween. Changing the distance between optic fiber ends in the cavity or stretching an optical fiber in the cavity changes the intensity of the combined optical intensity due to interference. The sensor can be designed to sense, for example, acoustic noise, stress/strain, temperature, vibration, shock etc.
The fibers must be very accurately aligned and able to maintain that alignment during operation. This is typically an expensive and timely process that involves either an active alignment or a passive alignment. The active alignment typically includes launching light into one fiber while maximizing the light coupled into the second fiber. The fibers are aligned in three planes and at a plurality of angles. In a conventional passive alignment, fibers are inserted into a small microcapillary tube and glued in place. These processes are also difficult to automate.
Thus, there is a need for a fiber optic Fabry-Perot interferometric sensor which is less expensive to produce. The process of making the interferometer should be easily automated and involve a less timely alignment of the fibers.